Lung cancer is the leading cause of cancer related mortality in both men and women and remains a major health issue. More than 162,000 individuals will die from lung cancer in the coming year, more than breast, prostate and colon cancer combined. The majority of lung cancer cases is attributable to tobacco smoking and in some cases other environmental risk factors. Although the relative risk of developing lung cancer declines dramatically in smokers who quit, former smokers remain at risk for the disease. Several recent studies show that greater than 50% of newly diagnosed lung cancers occur in former smokers. It is estimated that there are approximately equal numbers of smokers and former smokers in the United States. Since smoking cessation is a major public health initiative, former smokers will increasingly account for a higher percentage of lung cancer cases. Thus, two high-risk population groups exist for lung cancer and improved disease management can be beneficial to both current and former smokers. Chromosome 9p genetic alterations occur early and often in lung cancer; frequently at the p16/CDKN2 locus. However, we identified a region of homozygous deletion on the short arm of chromosome 9p at the marker D9S126 and proposed the existence of a tumor suppressor gene important in lung tumorigenesis in this region. We continued our analysis using 30 non-small cell lung cancer and 12 small cell lung cancer cell lines by screening with 55 genetic markers to identify new regions of homozygous deletion on chromosome 9p. Three novel non-contiguous homozygously deleted regions were detected. One of these regions at D9S126 led to the identification of a gene identified as TUSC1. Multiplex PCR and Southern blot confirmed the homozygous deletion of TUSC1 and Northern blot analysis of TUSC1 demonstrated two transcripts of approximately 2 and 1.5 kb that are likely generated by alternative polyadenylation signals. Both transcripts are expressed in several human tissues and share an open reading frame encoding a peptide of 209 amino acids. Analyzing lung cancer cell lines for RNA and protein expression demonstrated down regulation of TUSC1 in several cell lines further suggesting TUSC1 may play a role in tumorigenesis. Studies using lung tumor cell lines stably transfected with TUSC1 show reduced proliferation in vitro and reduced tumor formation in vivo. Taken together, these data suggest that chromosome 9p may contain other tumor suppressor genes important in lung tumorigenesis and that TUSC1 may be a candidate tumor suppressor gene. Current studies will employ yeast two hybrid experiments to determine potential protein binding partners for TUSC1. These interactions will be validated using immunoprecipitation and will lead to information concerning the pathways in which TUSC1 interacts. Antibody development for TUSC1 will provide the necessary reagents to examine multiple human lung tumors and determine the expression levels in the primary tumors. The expression levels can then be correlated to clinical outcome to further establish TUSC1 as a tumor suppressor gene.